Question

Give the precise thermochemical equation to which the following statements refer.
(i) The standard enthalpy of formation Of methane is - 74.8 K J/mol
(ii) The mean bond energy of C H bond in methane is 413 KJ/mol
(iii) The enthalpy of atomization of carbon is 715 KJ/mol
The molar enthalpy of combustion of carbon, hydrogen and buta-l ,
3-diene (CH2 = CHCH = CH2) are - 394, - 288 and - 2542 KJ/mol respectively. By
using a suitable thermochemical diagram or otherwise find the enthalpy of formation of buta-1, 3-diene.

Answer 1

C + 2H₂$\to$ CH₄ $∆H \stackrel{°}{f}$ =-78.8kjmol^-1

CH₄ $\to$ C + 4H. $∆H \stackrel{°}{f}$ =+1652kj mol^-1

C_(s) $\to$ C_(g) $∆H \stackrel{°}{f}$ =+715kj mol^-2

Let the enthalpy of formation of C₄H₆ be un unknown value such as x.

Using Hess law

$∆H \stackrel{°}{f}$ (C₄H₆)+$∆H \stackrel{°}{c}$ (C₄H₆)=$4×∆H \stackrel{°}{c}$ (C)+3$×∆H \stackrel{°}{c}$ (H)

X=(-394×4)+(-288×3)-(-2542)

=+102KJ mol^-1